Many motor vehicles can be modified for wheelchair accessibility and there are a variety of vendors throughout the U.S. that can provide this service. A quick search on the Internet will prompt a variety of companies that can help.
While shopping, though, keep in mind that when a motor vehicle is modified for wheelchair accessibility, the vehicle has to be certified to ensure that it still meets the US Motor Vehicle Safety Standard. Modifications must be performed by licensed manufacturers. Be sure the vendors you are shopping are licensed for this type of manufacturing.